1. Technical Field
The present invention relates to a pseudorange calculation method, a position calculation method, a computer-readable recording medium, and a position calculation device.
2. Related Art
The GPS (Global Positioning System) is widely known as a positioning system utilizing artificial satellites and is used for positioning devices built in portable telephones, car navigation devices and so on. In the GPS, positioning operation is carried out in which the values of four parameters, that is, three-dimensional coordinate values indicating the position of a user's machine and a timer error, are found on the basis of information including the positions of plural GPS satellites and the pseudorange from each GPS satellite to the user's machine. Thus, the current position of the user's machine is measured.
GPS satellite signals sent from GPS satellites are modulated by a spread code called PRN code, which is different from one GPS satellite to another. It is known that the PRN code has its polarity inverted at intervals of 20 milliseconds by phase modulation based on navigation data, for example, as disclosed in JP-A-11-258326.
To extract and capture a GPS satellite signal from a received RF signal, the received signal (or a signal after conversion to an intermediate frequency signal) is filtered so that an unnecessary frequency component is removed and reception sensitivity is thus improved. Particularly, since a GPS satellite signal is sent with a prescribed frequency, the pass band of the filter may be narrowed to extract only the vicinity of this frequency (or the frequency of the GPS satellite signal in the case where the received signal is converted to an intermediate frequency signal), thus improving reception sensitivity.
However, even if the frequency of a signal sent from a GPS satellite is a prescribed frequency at the time of sending, the frequency of the signal received by a GPS receiver is not the prescribed GPS frequency provided at the time of sending because of Doppler shift due to Earth orbiting of the GPS satellite, the influence caused when the signal passes through the ionosphere and the atmosphere, the influence of multi-path and so on. Therefore, if the pass band of the filter is made too narrow, the GPS satellite signal modulated by spread spectrum modulation is partly lost, causing a problem of lowered positioning accuracy. Of course, other satellite positioning systems than the GPS have similar problems.